in viexmng Objects with a Telescope. 509 



plane, or in any plane parallel to that, have a single vanishing 

 point; but the act of magnifying increases the distance of the 

 vanishing point in the same proportion as it does the ap- 

 parent dimensions of the object; consequently the magnified 

 object is not seen in true perspective for its own plane. 



Edinburgh, March 13, 1840. JaMES D. Forbes. 



Postscript. — A casual circumstance brought to my recol- 

 lection a few days ago an optical illusion mentioned (if indeed 

 it was not shown) to me some years since by Sir David Brew- 

 ster, the nature of which I could not perfectly I'ecollect. 

 Having applied to Sir David Brewster, he obligingly referred 

 me to the Edinburgh Journal of Science * for a notice of it, 

 when it proved, as I expected, to be referable to the principle 

 I have just been applying. 



" A field," says Sir David, " may be so situated, that " 

 (from the perspective of the furrows or drills upon its sur- 

 face) " when seen through the telescope it appears like a per- 

 pendicular or vertical voall of earth. This phasnomenon we 

 have often seen in directing a telescope to a field above Mel- 

 rose Abbey on the northern acclivity of the north-west 

 Eildon Hill. This field is capable of being ploughed in the 

 direction of its greatest declivity ; but when it is viewed 

 through a telescope, the slope is such that the furrows do not 

 appear to converge^ and the eye cannot readily perceive any 

 difference between die breadth of the furrows at the remote 

 end of the field and their breadth at the near end. The ob- 

 server therefore immediately concludes that the field must be 

 nearly in a vertical plane rising in front of him. This de- 

 ception is a very remarkable one, and produces a singular 

 effect on the mind wfien the field is covered with a crop, and 

 when crows, &c. light upon it." 



A more perfect illustration of the second form of the optical 

 illusion which I have described could not be desired. Every 

 one knows how imperfectly the eye estimates the acclivity of 

 a plane in full view. The parallelism of the ridges is tacidy 

 assumed, and as their apparent convergence diminishes ex- 

 actly in proportion as the magnifying power of the telescope 

 increases, the mind is forced to the conclusion that the plane 

 is more nearly perpendicular to the line joining the eye and 

 any point of it, than it really is. 



Hence it appears that Sir David Brewster noticed and 

 published fourteen years ago one case of the curious obser- 

 vation of Mr. Whitwell. 



March 17, 1840. 



* First Series, iii. p, 88, 



